Alignment dependence of high harmonics contributed from HOMO and HOMO-1 orbitals of N₂molecules

Abstract

Characteristics of high harmonics generated from aligned N 2 molecules were investigated by rotating the molecules with respect to the polarization of harmonic generation pulses. The high harmonics in the plateau and the cutoff regions showed distinctive behaviour, which could be explained from the calculation of transition dipole moments between recolliding electron and multi-orbitals of N 2 molecule consisting of the highest occupied molecular orbital (HOMO) and the lower-lying orbital below HOMO (HOMO-1). It was also observed that the angular dependence of harmonics from aligned molecules with respect to the polarization of harmonic generation pulses was closely related to the geometrical structure of molecular orbitals, and that the harmonic in the transition region from the plateau to the cutoff was influenced comparably by both HOMO and HOMO-1. These results confirmed that high harmonic generation and molecular structure were closely linked.

Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N2molecules

“…We revisit the issue about the importance of the HOMO-1 contribution raised in Refs. [13,22,27]. We find that HOMO-1 does not contribute to the HHG spectra at low laser intensities, but becomes comparable to the HOMO at higher intensities.…”

Theory of Nonlinear Propagation of High Harmonics Generated in a Gaseous Medium

“…The contribution of the π orbital to the HHG of N 2 molecules when the pump-probe angle α=90 • has been studied previously [163,177] using 800-nm pulses at the high intensities of around 2.0×10 14 W/cm 2 . In this case, HOMO-1 was found to be much more pronounced in the cutoff region.…”

“…This would make the retrieval of the structure of the target easier. If one wishes to study the π orbital, then a higher laser intensity and better alignment will enhance its contribution to the HHG [163,177].…”

“…The general trend of their data is very close to our prediction, but they used a higher intensity and a higher degree of alignment ( cos 2 θ =0.66), and thus the HOMO-1 channel may contribute to the signal at larger alignment angles. Using 800-nm lasers, Lee et al [177] reported the HHG ratios of aligned vs unaligned N 2 at the selected alignment angles. They did not present data near the resonance region and they used a high intensity of 2.5×10 14 W/cm 2 [see their Fig.…”

“…[103,163,177]. And then we will check how the famous shape resonance in HOMO PICS of N 2 present in the HHG spectrum.…”